i = int(input())
count = int(0)
sum = int(0)
while count < i:
sum = sum + count
count = count +1
print(sum)
aSA9IGludChpbnB1dCgpKQpjb3VudCA9IGludCgwKQpzdW0gID0gaW50KDApCgp3aGlsZSBjb3VudCA8IGk6CiAgICAgICBzdW0gPSBzdW0gKyBjb3VudAogICAgICAgY291bnQgPSBjb3VudCArMSAKCnByaW50KHN1bSk=